Controlling vacuum drying



Sept. 15, 1942. c, J. MERRIAM 2,295,744

CONTROLLING VACUUM DRYING Filed March 29,1940

frarZe zL/Zerrz'azn Patented Sept. 15, 1942.

UNITED STATES. PATENT OFFICE ooivraomive vacuum name Charles J. Merriam,Winnetka, 111., assignor to i'llllliei Guardite'corporation, acorporation oi Ilo s I Application March 29, 1940, Serial No. 326,721

2 Claims.. (01. 34-15) This invention relates to the control of vacuumdrying and more particularly to vacuum drying in which the drying mediumis superheated steam.

Indrying materials, for example, organic products such as rayon,tobacco, and the like, under vacuum conditions; one of the difllcultiesinvolved is the control of the flnal moisture content. In view of themaintenance of a vacuum, the taking of samples is difficult, and onaccount of the fact that the initial moisture content may vary widely indiflerent samples of the same product and even throughout the same batch,under treatment, it is diflicult to estimate the required treatmentfrom experience. -Moreover,

where the product has different moisture contents in diflerent parts,certain types of vacuum drying with superheated steam will not producethe necessary equilibrium, as for example, where a steady steamingoperation is employed rather than a succession of cycles of steaming andevacuation. 1

It has now been discovered that these difilculties may be overcome 'tothe extent that the initial moisture content of the product can bedisregarded as well as any variations in water content throughout thebody under treatment.

The process of the present application depends upon the utilization ofthe relationship between water vapor pressure and temperature andmoisture content of any given product. It is of course known that everyhygroscopic product will exert a given water vapor pressure at aparticular temperature. Applicant, however, was the first to realizethat this known fact could be when it reached that temperature itsmoisture content would be 10% regardless of how much moisture theproduct had at the beginning of the treatment.

when stated in this sirspi ejorm the above proposition appears to be aconverse hypothesis 'which should have been easily ascertainable by theart. Nevertheless, it was never grasped by anyone skilled in the art.Probably .this is because engineers have been confused by the dineringamounts of heat input required for different amounts of moistureremoved, and by the difierent amounts or heat input required where theobjects have diiierent starting temperatures.

It is also confusing because the proposition is true .30 only where theproduct is allowed to be at equi- The interposition of evacuation cyclesdoes not disturb the principle or operation of the above process.

The invention is illustrated diagrammatically,

so far as it is possible to illustrate the invention,

in the drawing in which l0 represents a container adapted to beevacuated, ii represents aline leading to evacuating means which are notperature than 140 F. The product is treated to remove all of the airandestablish an atmosphere of steam. This likewise is an important part orthe process because, if any air is present, all of the calculations arethrown oil. In hrder toremove the air it is preferred to follow theprocess oi the patent of applicant and Russell Wiles;

container is also provided with a pressure gauge t5 and one or morethermometers are main- 2,080,179. During that process the pressure onthe product is lowered far below 6" audits temperature will be alsoreduced. Thereafter a steam pressure is established on the product of6". inasmuch as the product has more than 10% oi moisture, it cansupport more than 6" at pressure at F. and, therefore, its temperaturein'the beginning will be below 140 F. and it will rise only as theproduct dries. The

heat tor drying may be. introduced in a. number 55 or ways andpreferably by superheated steam.

The introduced heat goes mostly toward remov- 'ing moisture and thetemperature rise of the product is very slow, but at each stage thetemperature of the product will correspond to a given moisture contentof the product under the particular pressure. By stopping at 140 F.,this content will be If the process were stopped at 135 F., it might beConversely, at 150 F., it might be 7%. Y

In practicing the process of this invention, the

initial moisture content of the material has proved to be of noimportance. For example, rayon having an initial moisture content of100% on a dry basis will dry to a moisture content 01 10% under the samefinal conditions as rayon having an initial moisture content of only Thediflerence is automatically taken care of by shortening of the time.That is, with a wet rayon the time necessary to increase the temperatureto a predetermined point under constant pressure conditions will bemuchgreater than with a dry rayon, but bothztypes will respond toprecisely the same control.

The exact temperature-pressure relationships vary with the type ofproduct undergoing treatment but maybe readily tested for it and, oncedetermined, may be used for regulating all future operations with thatmaterial.

For example, when rayon is employed, a wet mass of rayon ofindeterminate moisture content is subjected to a vacuum to removenon-condensible gases. 'Superheated steam is then introduced at aconstant sub-atmospheric. pressure and this is continued until thetemperature of the product reaches the temperature corresponding withthe desired final 5 moisture content. This will be the same regardlessof the initial moisture content.

Instead of'starting with a predetermined pressure and maintaining ituntil the temperature has risen to' a predetermined point, the productmay be brought to the desired final temperature and the pressuregradually lowered to maintain the temperature. This method is not asdesirable as the one first outlined because with most products theimposition oi the high temperature should be for as short a time aspossible. Furthermore, the control of the system is not as easy to carryout manually. Automatically, it may of course be taken care of byappropriate instruments.

It is of course possible to vary both temperature and pressure to reachthe desired conjunction of the two, but from an operating standpointthis is less desirable than either of the others, since it involves twovariables.

-The foregoing detailed description has been given for 'cleamess ofunderstanding only, and no unnecessary limitations should be understoodtherefrom.

What I claim as new, and desire to secure by Letters Patent, is:

1. Themethod of drying a hygroscopic product which comprisessubstantially removing noncondensible gases therefrom, heated steam overthe product at a selected subatmospheric pressure, the temperature ofthe steam being higher than the temperature of the product, andcontinuing passage of the steam over the product until the temperatureof the product reaches a predetermined value below that of the steam, atwhich temperature the vapor pressure of the moisture in .the product isin equilibrium with the steam pressure and at which temperature theproduct retains'a desired percentage of moisture as determined from avapor pressure curve of the product at the selected temperature. 2. Themethod as set forth in claim 1 in which .the product is rayon.

' 7 CHARLES J. MERRIAM.

passing super-.

